What Greek title meaning 'king' did Byzantine emperors adopt?

The Greek title meaning “king” that Byzantine emperors adopted was Basileus. The word originally denoted a king or monarch in Greek-speaking societies, especially during the Hellenistic period.

Roman emperors in the Greek-speaking East were often called basileus informally, but the title became official imperial language under Emperor Heraclius. In 629, after the Byzantine state had become increasingly Greek in language and administration, Heraclius used basileus in place of older Latin imperial formulas. It eventually appeared in the style basileus and autokrator of the Romans.

A common mix-up is Autokrator, which means “self-ruler” or “autocrat” and translated the Roman title imperator. Caesar and Augustus were also imperial titles inherited from Roman political tradition, but neither means “king” in Greek. Basileus later came to mean “emperor” in Byzantine usage, even though its basic Greek meaning remained “king” or “sovereign.”